Typically, silicon solar cells have both front- and back-side metallizations (front and back electrodes). A conventional silicon solar cell structure with a p-type base uses a negative electrode to contact the front-side or sun side of the cell, and a positive electrode on the back-side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate electron-hole pairs in that body. The potential difference that exists at a p-n junction, causes holes and electrons to move across the junction in opposite directions, thereby giving rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts which are electrically conductive.
The majority of the solar cells currently produced are based upon crystalline silicon. A popular method for depositing electrodes is the screen printing of metal pastes.
PERC silicon solar cells are well-known to the skilled person; see, for example, P. Choulat et al., “Above 17% industrial type PERC Solar Cell on thin Multi-Crystalline Silicon Substrate”, 22nd European Photovoltaic Solar Energy Conference, 3-7 Sep. 2007, Milan, Italy.
PERC silicon solar cells represent a special type of conventional silicon solar cells; they are distinguished by having a dielectric passivation layer on their front- and on their back-side. The passivation layer on the front-side serves as an ARC (antireflective coating) layer, as is conventional for silicon solar cells. The dielectric passivation layer on the back-side is perforated; it serves to extend charge carrier lifetime and as a result thereof improves light conversion efficiency. It is desired to avoid damage of the perforated dielectric back-side passivation layer as much as possible.
Similar to the production of a conventional silicon solar cell, the production of a PERC silicon solar cell typically starts with a p-type silicon substrate in the form of a silicon wafer on which an n-type diffusion layer (n-type emitter) of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the gaseous phosphorus diffusion source, other liquid sources are phosphoric acid and the like. In the absence of any particular modification, the n-type diffusion layer is formed over the entire surface of the silicon substrate. The p-n junction is formed where the concentration of the p-type dopant equals the concentration of the n-type dopant. The cells having the p-n junction close to the sun side, have a junction depth between 0.05 and 0.5 μm.
After formation of this diffusion layer excess surface glass is removed from the rest of the surfaces by etching by an acid such as hydrofluoric acid.
Next, a dielectric layer, for example, of TiOx, SiOx, TiOx/SiOx, SiNx or, in particular, a dielectric stack of SiNx/SiOx is formed on the front-side n-type diffusion layer. As a specific feature of the PERC silicon solar cell, the dielectric is also deposited on the back-side of the silicon wafer to a thickness of, for example, between 0.05 and 0.1 μm. Deposition of the dielectric may be performed, for example, using a process such as plasma CVD (chemical vapor deposition) in the presence of hydrogen or sputtering. Such a layer serves as both an ARC and passivation layer for the front-side and as a dielectric passivation layer for the back-side of the PERC silicon solar cell. The passivation layer on the back-side of the PERC silicon solar cell is then perforated. The perforations are typically produced by acid etching or laser drilling and the holes so produced are, for example, 50 to 300 μm in diameter. Their depth corresponds to the thickness of the passivation layer or may even slightly exceed it. The number of the perforations lies in the range of, for example, 100 to 500 per square centimeter.
Just like a conventional solar cell structure with a p-type base and a front-side n-type emitter, PERC silicon solar cells typically have a negative electrode on their front-side and a positive electrode on their back-side. The negative electrode is typically applied as a grid by screen printing and drying a front-side silver paste (front electrode forming silver paste) on the ARC layer on the front-side of the cell. The front-side grid electrode is typically screen printed in a so-called H pattern which comprises thin parallel finger lines (collector lines) and two busbars intersecting the finger lines at right angle. In addition, a back-side silver or silver/aluminum paste and an aluminum paste are applied, typically screen printed, and successively dried on the perforated passivation layer on the back-side of the p-type silicon substrate. Normally, the back-side silver or silver/aluminum paste is applied onto the back-side perforated passivation layer first to form anodic back contacts, for example, as two parallel busbars or as rectangles or tabs ready for soldering interconnection strings (presoldered copper ribbons). The aluminum paste is then applied in the bare areas with a slight overlap over the back-side silver or silver/aluminum. In some cases, the silver or silver/aluminum paste is applied after the aluminum paste has been applied. Firing is then typically carried out in a belt furnace for a period of 1 to 5 minutes with the wafer reaching a peak temperature in the range of 700 to 900° C. The front electrode and the back electrodes can be fired sequentially or cofired.
The aluminum paste is generally screen printed and dried on the perforated dielectric passivation layer on the back-side of the silicon wafer. The wafer is fired at a temperature above the melting point of aluminum to form an aluminum-silicon melt at the local contacts between the aluminum and the silicon, i.e. at those parts of the silicon wafer's back-surface not covered by the dielectric passivation layer or, in other words, at the places of the perforations. The so-formed local p+ contacts are generally called local BSF (back surface field) contacts. The aluminum paste is transformed by firing from a dried state to an aluminum back electrode, whereas the back-side silver or silver/aluminum paste becomes a silver or silver/aluminum back electrode upon firing. Typically, aluminum paste and back-side silver or silver/aluminum paste are cofired, although sequential firing is also possible. During firing, the boundary between the back-side aluminum and the back-side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode. A silver or silver/aluminum back electrode is formed over portions of the back-side as an anode for interconnecting solar cells by means of pre-soldered copper ribbon or the like. In addition, the front-side silver paste printed as front-side cathode etches and penetrates through the ARC layer during firing, and is thereby able to electrically contact the n-type layer. This type of process is generally called “firing through”.